Radio altimeters for aircraft are required to measure altitudes (range) from over 6000 feet to less than 4 feet. Single antenna frequency modulated continuous wave (FMCW) altimeter systems have short range limitations due to short path transceiver leakage through various paths that create self-interference. A short path transceiver leakage signal is primarily the result of combining a leakage signal due to the finite directivity of a transceiver circulator and reflections of the transmitter signal from (off of) the antenna into the receiver. The leakage signal due to the finite directivity of the transceiver circulator is referred to herein as a “circulator-leakage signal”. The reflection of the transmitter signal from the antenna into the receiver is referred to herein as an “antenna-reflection-leakage signal”.
The superposition of the circulator-leakage signal and the antenna-reflection-leakage signal at the receiver generates a self-interference, which can mask actual short range target returns. The circulator-leakage signal and the antenna-reflection-leakage signal typically have slightly different time delays through their different leakage paths. When the circulator-leakage signal and the antenna-reflection-leakage signal coherently recombine at the receiver mixer, they form a composite-leakage signal that has the same frequency but varies in phase and amplitude versus frequency. The composite-leakage signal is converted in the receiver mixer to a very low frequency difference signal resulting from the detection of both the phase modulation and amplitude modulation of the composite-leakage signal. If the target range is small, for example, 3 to 4 feet, the intermediate frequency (IF) of the target reflection signal is slightly higher than the IF of the composite-leakage signal and the altimeter system does not have sufficient frequency resolution to separate them into separate signals. The receiver mixer output signal is distorted resulting in range errors, missed detections, or grossly false range determinations.
Thus, the self-interference from the short path transceiver leakage signal is a significant limiting factor for ultra-short range performance.